Answer: The fill in the blank can be filled with Stressor.
Stressor is a condition or an event which causes strain or stress in the body of an organism. It can include any external stimuli, biological or chemical agent, environmental condition or any event which has potential of producing stress response.
Example- A tideous task provided to the employee in the company. If the time limit is too short and it actually requires double the time, it may cause strain on the body of the employee. Thus, the task acts a stressor.
Thus, correct answer of given question is stressor.